I assume you are refering to one of the earliest incidents in this book, when Junior throws the geometry textbook he is given at Mr. P. It isn't that Junior has something against the curves in geometry. Junior actually says that he was an intelligent student who loved learning in every subject. What made him react in such an angry and terrible way to Mr. P., the harmless and funny teacher, was that when he opened his geometry textbook he saw that it was the same book that his mother had had when she had studied at this school. Note what Junior thinks about this:

My school and my tribe are so poor and sad that we have to study from the same dang books our parents studied from. That is absolutely the saddest thing in the world.

Junior goes on to say that seeing his mother's name in the front of his textbook hit his heart like "a nuclear bomb" and that all of his hopes in life blew up with it. Junior doesn't hate the curves in geometry, or even geometry. He hates the poverty that is part of the daily reality of his existence as an Indian on a reservation.